Bug 689537 - (cov#10608) Fix Coverity NULL pointer dereferences
    
    It is possible that we dereference a NULL pointer in memberOf for
    a MODRDN. We should fix this to be safe, though I think it is
    extremely unlikely that pre_dn and post_dn will be NULL. We should
    check if these variables are NULL before calling
    memberof_replace_dn_from_groups().